The present invention pertains to a television transmitter. More particularly, the present invention pertains to a television transmitting system in which the frequency of the transmitter is controlled by reference signals common to several transmitters within a geographic region, thereby eliminating interference between two stations which are transmitting on the same frequency and which are geographically closely spaced.
The present invention can be utilized with any television transmitters but is particularly suited for use with multipoint distribution service (MDS) television transmitting systems which utilize low power transmitters to transmit signals within a limited geographic area. MDS transmitters in various geographic locations are frequently assigned to transmit on the same frequency or channel. This likewise might be true of other commercial and non-commercial television stations, such as stations in the Instructional Television Fixed Service (ITFS), particularly in large, overlapping metropolitan areas, for example San Francisco, Calif. and its suburbs. No problem exists if the transmitters are geographically spaced at a great enough distance that they do not interfere with each other in any location. However, the signals from two such transmitters located in relatively closely spaced urban locations might overlap in an area between the two transmitters. By way of example, the signals from a transmitter located in San Francisco and another located in Palo Alto, Calif. might both be receivable in a location between those two cities. Theoretically, the problem of interference in such an overlap area could be minimized if the two transmitters are transmitting at precisely the same carrier frequency. As a practical matter, however, slight frequency differences may exist in the signals from the two transmitters, and in the overlap area such frequency differences result in picture quality degradation.
Slight differences in frequency result in the generation of a beat frequency signal of a frequency equal to the difference in the carrier frequencies. The strength of the beat frequency signal is related to the difference in signal strength between the two carriers as seen at the input terminals of the receiving television set. The beat frequency signal can be detected by the average viewer whenever the difference between the signal strength of the desired signal and the signal strength of the undesired signal is less than a minimum amount, generally about 45 dB.
Observer sensitivity to the beat frequency signal is a function not only of the signal strength but also of the beat frequency. If the beat frequency can be reduced to a frequency below the lowest video frequency of the video signal, it can be effectively eliminated by circuitry within the television receiver, such as automatic gain control circuitry. Such circuitry can substantially reduce signal strength of a beat frequency of less than about 10 Hz. The video carrier frequency of an MDS Channel 1 station is 2154.75 MHz with a tolerance of 0.001%. In order to operate two stations on Channel 1 with a frequency difference of less than 10 Hz, the frequency determining elements of the transmitters would have to have tolerances of better than 2 parts in 1.sup.9. Operating such a system without a common reference would require the use of very precise frequency generators, such as very costly atomic frequency generators, and would be highly impractical.